Mauna Kea scopes out dwarf galaxies

Scientists say that the stars are bound together by gravity

WAIMEA, Hawaii » Using the giant Keck II telescope on Mauna Kea, astronomers have proved that at least eight nearly invisible clumps of stars around our Milky Way galaxy are dwarf galaxies acting like satellites of the Milky Way.

The groups of stars had been seen only in recent decades, and no one knew if they were bound together by gravity or are just random collections of stars, said Marla Geha, an astronomer with the Herzberg Institute of Astrophysics in Victoria, British Columbia, Canada.

Geha's team discovered that gravity keeps the stars buzzing around each other like bees, she said. That's different from the Milky Way, where most of the stars are spinning around the galactic center in an orderly way.

Geha's discovery can best be understood by anyone who has heard of two large dwarfs called the Large and Small Magellanic Clouds, visible from the Southern Hemisphere.

The dwarfs Geha studied are as much as 1,000 times smaller -- "tiny," she said.

They're also much darker. In fact, they are composed of 99 percent mysterious dark matter. It's invisible in any wavelength of light astronomers look at, yet it must be there because the effects of its gravity can be seen.

"If you had asked me last year whether galaxies this small and dark existed, I would have said no," Geha said. She said she was "astonished" by the discovery.

Geha and colleague Joshua Simon of the California Institute of Technology looked at the speed, caused by gravity, of 814 stars in eight dwarf galaxies.

The stars turned out to be slowpokes, moving a bit more than 2 to 4 miles per second. In comparison, our sun is moving about 140 miles per second as part of the galactic spiral.

Although about two dozen such dwarf galaxies have been seen, the current Cold Dark Matter theory says there should be about 1,000 around the Milky Way.

There's a good chance that most of those don't have any stars, just that irritating dark matter that no one can see.

"If the Cold Dark Matter model is correct, they have to be out there, and the next challenge for astronomers will be finding a way to detect their presence," Geha said.